Capacitor banks are installed to improve the quality of an electrical supply by providing reactive power compensation and power factor correction in a power system. The use of capacitor banks has increased because they are relatively inexpensive, easy and quick to install, and can be deployed almost anywhere in a power system grid. Capacitor bank installations have other beneficial effects on the system such as improvement of the voltage profile, better voltage regulation, reduction of losses, and reduction or postponement of investments in the transmission and generation capacity.
A capacitor bank is assembled by a plurality of individual capacitor units. Each individual capacitor unit is a basic building block of the capacitor bank and includes a plurality of individual capacitor elements, arranged in parallel/series connected groups, within a metal enclosure. The units can be externally or internally fused, fuseless or unfused depending on the application of the bank. The elements can be connected to fuses and a group of elements is normally shunted by an internal discharge resistor in order to reduce the unit residual voltage after being disconnected from the power system. Each of the capacitor elements is constructed by winding two electrodes of aluminum foil separated by several layers of paper, or plastic film-insulated or a mixed dielectric of paper and plastic film.
Capacitor banks are normally further constructed using individual capacitor units connected in series and/or parallel to obtain a required voltage rating.
However, an internal failure in terms of operated fuses, failed elements and/or failed units in one or more quadrants may occur due to, for example the improper selection of the designed voltage rating that may result in continuous high voltage stress for the selected capacitor bank and eventually may lead to a dielectric failure of capacitor elements. Other causes for internal failure can be overcurrent, overvoltage, short-circuit, thermal failure and internal stress. It may also include incorrect operation of fuses due to bad fuse coordination.
Existing unbalanced protection schemes are typically available to detect such an internal failure. For example, unbalance protection can be utilized in a variety of capacitor bank connections: grounded Y, ungrounded Y, delta, and single-phase. For example, a Y-Y arrangement is a preferable configuration for unbalanced protection of a large capacitor bank that is split into two Y sections. This scheme is based on a current measurement, for example, a current transformer, arranged between the two neutrals. Any change in the capacitance of any capacitor will cause a change in the current measurement.
However, the existing unbalanced protections based on a double Y scheme are liable to detect the number of element and/or unit failures incorrectly. For example, in a three-phase capacitor bank, an internal failure in one leg may cancel the unbalanced signal produced by another internal failure in another leg sharing the same phase but in the other section of the bank, while failures involving three legs in the same Y section may results in no unbalanced signal. Consequently, neither alarm nor trip signal would be sent when it should be sent. Undetected failures inside a capacitor bank may lead to a risk of fire or explosion accompanied by severe damage to the whole capacitor bank before taking early preventive action.
Furthermore, failures in two phases of one Y section may be seen as a failure in the third phase of the other Y section, which may result in an over-accumulation effect as if there is already a failure in this third phase. This over-accumulation effect may further result in a false alarm or trip signal.
In “Principles of shunt capacitor bank application and protection”, by SATISH SAMINENI ET AL published in PROTECTIVE RELAY ENGINEERS, 2010 63RD ANNUAL CONFERENCE FOR, IEEE, PISCATAWAY, N.J., USA, 29 Mar. 2010 (2010-03-29), pages 1-14, XP03f679144, ISBN: 978-1-4244-6073-1.(D1) some methods of fault detection of capacitor banks are disclosed.
D1 especially describes a method for detecting a fault in an ungrounded “double wye” capacitor bank. D1 discusses determining the phase of the capacitor bank where a fault occurs and the number of faulty units in the phase. The method of D1 uses a positive sequence current as a reference for identifying in which phase a fault occurs. The angle of the measured unbalance current of the neutral interconnection between the two parallel “WYEs” are determined in relation to the symmetrical positive sequence current. 0 degrees indicates a fault in phase A, 60 degrees indicates phase B, 120 degrees phase C, 180 degrees phase A, 240 degrees phase B and 300 degrees indicates phase C. However, after a first fault has occurred, the currents will no longer be symmetrical, and the method does not discuss how to find subsequent faults following the first fault. The method of D1 is limited to finding the faulty phase of one fault event and leaves no clue how to identify subsequent faults.
Therefore, a more sensitive and accurate internal failure detection and protection scheme is highly desired.